Uninterruptible power supplies or systems (commonly referred to as UPS) are used to provide back-up power to critical loads such as computer systems where a loss of line power can result in the interruption of programs and the loss of valuable data. Uninterruptible power supplies may also provide a line power conditioning function to ensure that transient spikes, low voltage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer which is supplied with power through the UPS. Typically, the UPS includes a battery which is interfaced through an inverter to the AC output line. One type of UPS operates such that when a fault occurs in the input AC power, the inverter is controlled to provide power from the battery to the AC output line at the same frequency and with substantially the same waveform as the normal input AC power. Preferably, the switching at the time of fault is accomplished as smoothly as possible so that substantial transient spikes or dips in the waveform supplied to the AC output line do not occur. The coupling of the converter to the AC output may be through a ferroresonant transformer as illustrated in U.S. Pat. No. 4,692,854 to Richard V. Baxter. et al. entitled Method and Apparatus for Modulating Inverter Pulse Width, the disclosure of which is incorporated herein by reference.
In double conversion UPS systems, the AC power is rectified to a DC voltage which is applied, in parallel with the battery voltage, to a constantly running inverter. If the inverter switching elements or the controls for the inverter fail, the consequence is an immediate failure (or distortion) of the power supplied to the critical load. In such cases, a bypass is usually provided around the UPS which is switched to supply line power directly to the load upon detection of failure of the inverter. In UPS systems which utilize a ferroresonant transformer and in certain other types of UPS systems, the inverter need not be running constantly and a failure of the inverter or its control circuitry will not result in an immediate disruption of power to the load as long as line power is available. Consequently, a failure of the inverter circuitry, or of ancillary circuitry such as the fuses provided in the inverter system, or of the battery, may not become apparent until a power outage occurs and power from the inverter is required. The operation of the inverter and the other circuitry of the UPS can, of course, be tested by periodically cutting off the line power and switching on the inverter to verify that the inverter is properly supplying power to the load. However, if the inverter or other components are not properly functioning, the result can be an unnecessary power interruption to the load. Consequently, it is desirable to test the inverter while the load is still being provided with power from the AC power system and without creating voltage transients or other waveform distortions in the power supplied to the load while the inverter is being tested.